Apparatus for electrodischarge machining

ABSTRACT

An electric discharge machining apparatus includes a first movable body ( 40, 42, 50 ) movable along the Z axis, a ball screw ( 34 ), a motor ( 30 ) for causing rotation of the ball screw, a nut ( 35 ), attached to the first movable body, threadingly engaging the ball screw, a second movable body ( 10, 13 ) movable along the Z axis relative to the first movable body, capable of having the tool electrode attached thereto, and a linear motor ( 71, 72, 73 ) for moving the second movable body, for machining a workpiece by moving a tool electrode along a Z axis towards the workpiece while causing an electric discharge between the workpiece and the tool electrode. The linear motor includes a stator ( 73 ) attached to the first movable body, and a mover ( 71, 72 ) attached to the second movable body. The first and second movable bodies include respective electrode attachment units ( 50, 13 ).

FIELD OF THE INVENTION

The present invention relates to an electric discharge machine formaking holes of various shapes in a workpiece by advancing a toolelectrode towards the workpiece while causing an electric dischargebetween the tool electrode and the workpiece.

BACKGROUND OF THE INVENTION

Electric discharge machines are widely used to accurately machine solidconductive workpieces into molds of various shapes. The workpiece isnormally placed in a work. tank and fixed to a table movable in thehorizontal plane. The tool electrode is attached to a lower end of aquill movable in the vertical direction using a suitable electrodeholder. The tool electrode is manufactured from a material that is easyto cut, such as, for example, copper or graphite. In preparation formachining the workpiece, the work tank is filled with dielectric fluidsuch as kerosene, and the tool electrode is positioned extremely closeto the workpiece. A space between the tool electrode and the workpieceis called a gap, and size of this gap is controlled to be between a fewμm to a few tens of μm. If a power pulse is applied between the toolelectrode and the workpiece during the ON-time, the insulatingcharacteristics of the dielectric fluid in the gap break down and anelectric discharge occurs. The material of the workpiece evaporates ormelts as a result of the heat from the electric discharge and becomesentrained in the dielectric fluid. Upon completion of the ON-time,application of the power pulse is suspended during the OFF-time and theinsulating properties of dielectric fluid in the gap are restored.Electric discharge machines ordinarily repeatedly apply the power pulseto the gap with the ON-time and the OFF-time controlled between 1 μsecto several tens of msec. As the gap is maintained at a constant size,the tool electrode is gradually moved downwards towards the workpiece inaccordance with removal of the workpiece material. In a coordinatesystem for positioning the tool electrode relative to the workpiece, aline representing an amount of linear movement of the tool electrodetowards the workpiece is called the Z axis from a control viewpoint. Inmany electric discharge machines, the Z axis number normally representsa position of the tool electrode in the vertical direction. Since theelectric discharge machine removes a microscopic amount of material fromthe workpiece at a time without the tool electrode actually coming intocontact with the workpiece, a cavity having a desired surface roughnessis formed in the workpiece with good accuracy. The cavity iscomplementary in size and shape to the tool electrode, which means thatvarious tool electrodes are used according to the shape of the cavityrequired. In order to make a large cavity, a large tool electrodebecomes necessary, and electric discharge machines capable of holding atool electrode in excess of 100 Kg on the quill are known.

A flushing operation for producing a flow of dielectric fluid throughthe gap is necessary in order to rinse fragments that have been removedfrom the workpiece from the gap. The flushing operation preventsundesirable secondary discharge between the tool electrode and fragmentsthat have been removed from the workpiece, and contributes torestoration of the insulating properties of the dielectric fluid duringthe OFF-time. In preparation for machining of the workpiece, a skilledoperator will make holes for introducing fresh dielectric fluid into thegap and suctioning used dielectric fluid from the gap in the toolelectrode and the workpiece. Flushing is the key to faster and betterprecision electric discharge machining, but skill and experience arerequired in order to produce uniform flow across the entire gapaccording to the shape of a required cavity. Depending on the situation,it may not be desirable to form flushing holes in the workpiece, orthere might be restrictions in forming those types of holes in the toolelectrode. For example, in the case where an operator is making a deepcavity having an elongated opening in a workpiece, a thin rib-shapedtool electrode is used. Because it is difficult to form flushing holesin such a tool electrode, an injection system is normally used to injectdielectric fluid from the side of the tool electrode towards the gap.However, an injection system can not sufficiently remove contaminateddielectric fluid from the gap as the cavity being formed in theworkpiece becomes deeper. An operation known as a “jump” is known forcompensating for this insufficient flushing operation. The jumpoperation involves periodically raising and then lowering the toolelectrode rapidly in the Z axis direction, and drives out almost all ofthe contaminated dielectric fluid from the cavity in the workpiece.Conventionally, the tool electrode moves at a speed of several hundredmm/min during the jump operation. If the reciprocating distance of thetool electrode is large, more fresh dielectric fluid flows in to thegap, and more contaminated fluid is removed from the gap. The toolelectrode is preferably raised up by at least the depth of the cavitybeing machined in the workpiece. However, since no material is removedfrom the workpiece during the jump operation, performing the jumpoperation too frequently will adversely lower the stock removal rate. Inorder to carry out a jump operation with a large amount of movement thatdoes not cause a lowering of the stock removal rate, the tool electrodeis preferably made to move at high speed and with an acceleration anddeceleration in excess of 1 G.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an electric dischargemachining apparatus that can effectively wash fragments removed from aworkpiece away from a gap without causing a reduction in stock removalrate, even when machining a deep cavity using a thin tool electrode.

Another object of the present invention is to provide an electricdischarge machine that is as compact as possible.

Additional objects of the invention will be set forth in the descriptionthat follows, and will become apparent to those skilled in the art uponpracticing the invention.

In order to achieve the above and other objects, an electric dischargemachining apparatus according to one aspect of the present invention formachining a workpiece by moving a tool electrode along a Z axis towardsthe workpiece while causing an electric discharge between the workpieceand the tool electrode comprises:

a first movable body movable along the Z axis;

a ball screw;

a motor for causing rotation of the ball screw;

a nut, attached to the first movable body, threadingly engaging the ballscrew;

a second movable body movable along the Z axis relative to the firstmovable body, capable of having the tool electrode attached thereto; and

a linear motor for moving the second movable body.

The linear motor preferably comprises a stator attached to the firstmovable body, and a mover attached to the second movable body.

The first movable body preferably comprises an electrode holder to whichthe tool electrode can be attached.

The second movable body is preferably provided coaxially with the firstmovable body.

In accordance with another aspect of the invention, an electricdischarge machining apparatus of the present invention for machining aworkpiece by moving a tool electrode along a Z axis towards theworkpiece while causing an electric discharge between the workpiece andthe tool electrode comprises:

a first movable body movable along the Z axis and having a firstelectrode holder to which the tool electrode can be attached;

a ball screw;

a motor for causing rotation of the ball screw;

a nut, attached to the first movable body, threadingly engaging the ballscrew;

a second movable body movable along the Z axis relative to the firstmovable body, and having a second electrode holder to which the toolelectrode can attached; and

a linear motor for moving the second movable body.

Accordingly, the tool electrode can be selectively attached to one ofthe first and second electrode holders. A detector is preferablyprovided for detecting selection of the electrode holder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional side elevation illustrating an embodiment ofan electric discharge machining apparatus of the present invention.

FIG. 2 is a vertical cross section, viewed from the front, of theelectric discharge machining apparatus of FIG. 1.

FIG. 3 is a horizontal cross section of the electric discharge machiningapparatus of FIG. 1 viewed along line A—A.

FIG. 4 is a bottom view of the electric discharge machining apparatus ofFIG. 1 viewed along line B—B.

FIG. 5 is a side elevation of the electric discharge machining apparatusof FIG. 1.

FIG. 6 is a cross section showing an enlarged view of a lower end of aframe of the electric discharge machining apparatus of FIG. 1.

FIG. 7 is a cross section showing the electrode detection unit of theelectric discharge machining apparatus of FIG. 1.

DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS OF THE INVENTION

Exemplary embodiments of an electric discharge machining apparatusaccording to the present invention will now be described with referenceto FIGS. 1, 2, 3, 4, 5, 6 and 7.

As illustrated in FIG. 1, an ac servo motor 30 provided with a rotaryencoder 31 and a brake 32 is fixed to a column 1. A ball screw 34extending in a vertical direction is rotated by the servo motor 30. Aball screw nut 35 which threadingly engages the ball screw 34 isattached to a frame 40. The frame 40 is a main part of a body movable inthe Z axis direction. A tool electrode or a tool holder may be attachedto the movable body. In the illustrated embodiment the frame 40 has asquare cross section and extends in the vertical direction. A pair oflinear motion bearing rails 61 for guiding the frame 40 movable in the Zaxis direction are attached to the frame 40 so as to be opposed withrespect to the ball screw nut 35.

Upper and lower linear motion bearing blocks 62 attached to the column 1are fitted into the respective rails 61. The rotary encoder 31 feedsback a signal representing the position of the frame 40 on the Z axis toan NC device (not shown). The NC device supplies a movement commandsignal to a drive unit of the servo motor 30. The frame 40 asillustrated in FIGS. 1, 2 and 5 is positioned at the upper limit of theZ axis.

As illustrated in FIG. 2, a plate 42 is attached to the lower end of theframe 40 by a number of bolts 45. A suitable electrode attachment unit50 is detachably attached to the plate 42.

As shown most clearly in FIG. 6, an insulating plate 41 is providedbetween the frame 40 and the plate 42 and insulating bushes 49 andinsulating washers 47 are provided between the plate 42 and the bolts45. Holes for insertion of the bolts 45 are formed in the plate 42, andblocked off by insulating covers 48.

As shown most clearly in FIG. 4, the plate 42 has an opening 43, and anumber of blocks 44 for accurately attaching the electrode attachmentunit 50 to the plate 42 are attached to the plate 42.

As best illustrated in FIG. 2, T slots are formed in the electrodeattachment unit 50 so that a large tool electrode or tool holder can beattached to the electrode attachment unit 50. A cable for transmittingpower to the tool electrode is connected from a power source, through aterminal box (not shown) provided on the column 1, to the electrodeattachment unit 50. A plate 81 extends horizontally from the column 1,and an opening 82 through which the body movable in the Z axis passes isformed in the plate 81. A bellows 83 for dust protection is providedbetween the plate 81 and the plate 42. A cover 9 surrounding the servomotor 30 and the frame 40 is attached to the column 1 and the plate 81.A workpiece is fixed on a table (not shown) moveable in the orthogonal Xaxis and Y axis directions which are vertical to the Z axis. A weight(not shown) for balancing a gravitational force acting on the frame 40is provided behind the column 1. As shown in FIG. 5, this weight isconnected to hangers 25 respectively attached to both sides of the frame40, using wire 26. As illustrated in FIG. 3, air cylinder units 28 areattached to respective brackets 29 fixed to the column 1. The aircylinder units 28 can fix the frame 40 to the column 1 by pressingpiston rods against the frame 40.

A hollow quill 10 is arranged inside the frame 40. This quill 10 is amain part of a movable body movable in another Z axis direction relativeto the frame 40. A tool electrode or tool holder can be attached to themovable body. The amounts of travel of the two parallel Z axes willprobably be different, but the two Z axes preferably overlap. The quill10 has a square cross section and extends vertically coaxially with theframe 40. A pair of linear motion bearing rails 63 for guiding the quillmovable in another Z axis direction are attached to the quill 10. Upperand lower linear motion bearing blocks 64 attached to the inner surfaceof the frame 40 are fitted into respective rails 63. A cylindricalspacer 12 is attached to a lower end of the quill 10 so as to hold adisk-shaped insulating plate 11 between the quill 10 and the spacer 12.An electrode attachment unit 13 having a chuck for holding a small toolelectrode or electrode holder is fixed to the spacer 12 coaxially withthe quill 10.

A cable for transmitting power to the tool electrode is connected from apower source, through a terminal box (not shown) provided on the column1, to the electrode attachment unit 13. The power source is selectivelyconnected to one of the electrode attachment units 13 and 50 usingswitches inside the terminal box. The electrode attachment unit 13comprises a servo motor for rotating the tool electrode centrally aroundanother Z axis, and a rotary encoder having high resolution and beingcapable of indexing the rotation angle of the tool electrode.

In order to perform a jump operation with a large amount of movementwithout lowering the stock removal rate, the body movable in another Zaxis is capable of moving with high acceleration through the opening 43in the plate 42 using two linear motors. The stators of the linearmotors comprise a magnetic plate 71 affixed to the side surface of thequill 10, and a plurality of permanent magnets 72 arranged in a row onthe magnetic plate 71. Each of the permanent magnets 72 is inclinedslightly from the horizontal direction in order to reduce torque ripple.The movers of the linear motors comprise an electromagnet 73 made up ofa coil and a yoke. Respective structural elements of the two linearmotors are arranged symmetrically about the central axis of the quill10. If current flows in coils of the electromagnets 73, the quill 10moves in a vertical direction due to thrust generated between thestators and the movers.

As clearly shown in FIG. 2, two electromagnets 73 are attached to plates74 maintaining size of the gap between electromagnets 73 and opposingpermanent magnets 72 at the same small value. The plates 74 arevertically fixed in windows 46 formed in the quill 10. An air cylinderunit 20 is provided for generating a balance force to counteract thegravitational force acting on the quill 10. A cylinder 21 of the aircylinder unit 20 is provided coaxially with the quill 10. An upper endof the cylinder 21 is fixed to the frame 40, and a locking device 23 isprovided on a lower end of the cylinder 21. One end of a piston rod 22is linked to a piston (not shown) sliding on an inner wall of thecylinder 21, and the other end of the piston rod 22 is linked to anupper end of the quill 10 by a suitable coupling. The pressure of airsupplied to the cylinder 21 is adjustable depending on the weight of thetool electrode etc. The air cylinder unit 20 can follow movement of thequill 10 with an acceleration in excess of 1 G with higherresponsiveness than a balancing weight. The air cylinder unit 20includes a locking device 23 through which the piston rod 22 passes. Thelocking device 23 prevents unexpected dropping of the tool electrode atthe time of electricity failure by gripping the piston rod 22. Inaddition, when movement of the quill 10 in the Z axis direction isnecessary, the locking device 23 fixes the quill 10 to the frame 40. Alinear scale 65 is fixed to one of the four sides of the quill 10 thatdoes not have a linear motor mover fixed thereto. A sensor 66 forreading out a position of the quill 10 on another Z axis is provided onan inner surface of the frame 40. The sensor 66 feeds back a signalrepresenting the position of the quill 10 on another Z axis to an NCdevice, and the NC device supplies a position command signal to a driveunit of the linear motor. The drive unit supplies a current signal tocoils of the electromagnets 73.

One of the two electrode attachment units 50 and 13 is used depending onthe shape of the tool electrode. When a deep cavity is to be machinedusing a thin tool electrode, the electrode attachment unit 13 is used.It goes without saying that when an effective flushing operation can notbe expected, a high acceleration jump operation using the linear motoris of significant assistance to the insufficient flushing operation.Alternatively, fragments that have been removed from the workpiece canbe effectively washed away from the gap by using a high accelerationjump operation instead of the relatively insufficient flushingoperation. In cases other than this, the electrode attachment unit 50 isused. Thus, since it is not expected that a large heavy tool electrodewill be attached to the electrode attachment unit 13, there is no needfor the linear motor to generate an excessively large thrust and theefficiency of the linear motor is not reduced due to over heating. Inorder to sufficiently exhibit the high acceleration and high speedperformance of the linear motor, another Z axis movable body comprisingthe quill 10 and the electrode attachment unit 13 is preferably made aslightweight as possible. An operator can select one of the two Z axesusing an input device of the NC device. With the illustrated embodiment,the NC device can recognize a selected Z axis using a signal from theelectrode detection unit 90.

As shown in FIG. 7, the electrode detection unit 90 comprises a pin 91having a lower end protruding from a lower surface of the plate 42, aspring 92 for pressing the pin 91 downwards, and a limit switch 93opposite to the upper end of the pin 91. If the electrode attachmentunit 50 is attached to the plate 42, the pin 91 engages the spring 92and presses an actuator of the limit switch 93. If the limit switch 93sends a signal indicating use of the electrode attachment unit 50 to theNC device, the NC device controls switches inside the terminal box toconnect the power source to the electrode attachment unit 50. The NCdevice also controls the locking device 23 to fix the quill 10 to theframe 40. If a signal indicating use of the electrode attachment unit 13is transmitted from the limit switch 93 to the NC device, the powersource is connected to the electrode attachment unit 13. The NC devicealso controls the air cylinder unit 28 to fix the frame 40 on the column1, and operates the brake 32 of the servo motor 30.

The present invention is not intended to be limited to the formdisclosed. Various improvements and modifications are clearly possibleupon reference to the above description. For example, although twoelectrode attachment units 50 and 13 are provided, if it is possible toattach tool electrodes of various sizes to the electrode attachment unit13, the electrode attachment unit 50 can be omitted. However, in caseswhere it is anticipated that large heavy tool electrodes will also beused, the two electrode attachment units 50 and 13 are preferablyprovided, as shown in the embodiment, in order to make the electrodeattachment unit 13 lightweight.

The illustrated embodiment has been selected in order to describe theessence and practical application of the present invention. The scope ofthe invention is defined by the attached claims.

What is claimed is:
 1. An electric discharge machining apparatus formachining a workpiece by moving a tool electrode along a Z axis towardsthe workpiece while causing an electric discharge between the workpieceand the tool electrode, comprising: a first body movable along the Zaxis; a ball screw; a motor for rotating the ball screw; a nut, attachedto the first body for threadingly engaging the ball screw; a secondbody, movable along the Z axis relative to the first movable body,adapted to accept a tool electrode; and a linear motor for moving thesecond body.
 2. The electric discharge machining apparatus of claim 1,wherein the linear motor comprises a stator attached to the first body.3. The electric discharge machining apparatus of claim 1, wherein thesecond body is positioned coaxially with the first body.
 4. An electricdischarge machining apparatus for machining a workpiece by moving a toolelectrode along a Z axis towards the workpiece while causing an electricdischarge between the workpiece and the tool electrode, comprising: afirst body move along the Z axis; a ball screw; a motor for rotating theball screw; a nut, attached to the first body for threadingly engagingthe ball screw; a second body, movable along the Z axis relative to thefirst body, adapted to accept a tool electrode; and a linear motor,comprising a stator attached to the first body and a mover attached tothe second body, for moving the second body.
 5. An electric dischargemachining apparatus for machining a workpiece by moving a tool electrodealong a Z axis towards the workpiece while causing an electric dischargebetween the workpiece and the tool electrode, comprising: a first bodymovable along the Z axis and having a first electrode holder adapted toaccept the tool electrode; a ball screw; a motor for rotating the ballscrew; a nut, attached to the first movable body for threadinglyengaging the ball screw; a second body, movable along the Z axisrelative to the first movable body, and having a second electrode holderadapted to accept a tool electrode; and a linear motor for moving thesecond body.
 6. The electric discharge machining apparatus of claim 5,wherein the first electrode holder includes a detector for detectingattachment of the first electrode holder to the first body.